Dragon 2 lacks the grapple point necessary for the Canadarm to grab it (unlike Dragon 1, which did indeed get berthed using the arm). Dragon 2 has already done

Dragon 2 lacks the grapple point necessary for the Canadarm to grab it (unlike Dragon 1, which did indeed get berthed using the arm). Dragon 2 has already done an autonomous docking to the ISS, as part of the DM-1 mission.